The trolley of a stacker crane runs on elevated transport rails, and the trolley itself comprises a pair of bridging rails on which a turntable is mounted, so that the turntable can be moved both parallel to the transport rails and transversely to them. The mast that projects downward from the turntable is thus movable horizontally in all directions as well as being rotatable about its own axis by rotation of the turntable.
In a typical installation, a stacker crane is used with racks that provide storage stations arranged in horizontal rows and vertical columns. Usually such racks are located on opposite sides of an aisle that extends parallel to the transport rails. The mast is moved along the racks to be brought opposite a vertical column that contains a storage station to be loaded or unloaded, and the load supporting means is moved vertically along the mast to a position at which that station is directly in front of it, whereupon the load supporting means must be moved forwardly into the station to accomplish loading or unloading.
In heretofore conventional stacker cranes, the load supporting means usually comprised a pair of forwardly projecting tongues, generally similar to the fork of a fork lift truck, mounted on one side of the mast to project outwardly from it. The load supporting means was moved into and out of a storage station by an appropriate horizontal movement of the mast.
Because the load supporting means of such a crane was disposed wholly at one side of the mast, the width of any aisle in which the crane had to perform loading and unloading operations had to be equal to the length of the load supporting means, plus the width of the mast, plus reasonable clearance. Since the loads to be handled by the crane were normally no wider than the length of the load supporting tongues, the width of an aisle between storage racks had to be substantially greater than would have been adequate for the dimensions of the loads themselves, the additional width being accounted for by the width of the mast, which could be on the order of three feet. Such additional aisle width, multiplied by the length of the aisle or aisles along which the stacker crane operated, amounted to a very significant number of square feet of floor area that had to be provided for no other purpose than to meet requirements of the stacker crane. In a building initially constructed to accommodate a stacker crane installation, the need for this additional floor area not only increased construction costs but also involved proportionate increases in maintenance costs, taxes and insurance. In a building already constructed, the advantages obtainable from installation of a stacker crane could be more than offset by sacrifice to the crane of floor space needed for other purposes.
Economic disadvantages were not the only ones imposed by the heretofore conventional stacker crane configuration. There were situations in which a stacker crane could not be used because of space limitations, even though use of such a crane would otherwise have been highly advantageous. Consider, for example, a shop having two rows of forming presses arranged at opposite sides of a relatively narrow aisle, each having an overhanging top portion that projects into the aisle, and each requiring an occasional change of heavy forming dies that can be stored in racks of the above described character. In this case--which is not an unusual one--the arrangement of the presses is mandated by rigid floor space limitations. Obviously it would be advantageous to employ a stacker crane for transporting the dies to and from the storage racks and moving them into and out of the bottom portions of the presses. But a stacker crane of prior configuration could not be used if the space between the bottom portions of opposite presses were just wide enough to accommodate the mast and the load engaging means, because the overhanging top portions of the presses would get in the way of the mast as it moved horizontally to carry the load supporting means to and from its loading and unloading position. Heretofore, in a shop such as just described, dies had to be moved into and out of each press manually, and several hours were needed for a die change that could have been accomplished in minutes if a stacker crane could have been used.